Method for tracking the position of an irradiating source

ABSTRACT

Method for producing a reconstruction image, the reconstruction image showing a position of irradiating sources in an environment, the reconstruction image being established on the basis of gamma images acquired by a gamma camera, which is sensitive to ionizing electromagnetic radiation, and movable relative to at least one irradiating source between two different measurement times, the gamma camera being joined to a visible camera, which is configured to form a visible image of the environment, the gamma camera and the visible camera defining an observation field, the method comprising establishing a reconstruction image, showing a position of at least one irradiation source in the observation field, the gamma camera and the visible camera being moved between at least two measurement times.

TECHNICAL FIELD

The technical field of the invention is the characterization ofirradiating sources present in an environment, notably in a nuclearinstallation or an installation comprising irradiating sources.

PRIOR ART

Gamma cameras are devices allowing, using an image, irradiating sourcesin a given environment, and in particular in nuclear installations, tobe mapped. This type of device was developed in the 1990s, and isincreasingly used in nuclear installations for the purposes ofradiological characterization. The objective is to identify the mainirradiating sources present in an installation. Specifically,irradiating sources are not uniformly distributed. They are oftenconcentrated locally, forming “hotspots” to use the term conventionallyused in the field of radioprotection. Gamma cameras are advantageous inthat they allow these hotspots to be located at distance.

The development and use of gamma cameras have been abundantly describedin the literature. Since the start of the 2000s, spectrometric gammacameras have been under development. These cameras are based on apixelated imager, each pixel allowing a spectrum to be obtained from theirradiation that it detects. Irradiating sources may be located far moreeasily as a result. Specifically, the spectrometric function allowsenergy bands of interest, corresponding to unscattered photons, i.e.photons that have not been deviated since their emission by theirradiation source, to be selected. The path of unscattered photons isstraight. Their selection, in predetermined energy bands, allows noisecorresponding to scattered photons to be removed. Since the latterphotons have been deviated since their remission, they provide no usefulinformation as to the location of the irradiating sources. Scattering istherefore a noise source that may be significantly limited byspectrometry.

Another advantage of spectrometric gamma cameras is that knowledge ofthe energy of the photons allows the isotopes responsible for theirradiation to be identified. This is information that is important inthe field of radioprotection, or in the management of radioactive waste,or even when dismantling nuclear installations, or performingradiological characterization after an accident.

One constraint associated with use of gamma cameras results from thefact that it is impossible to focus X-rays or gamma rays the energy ofwhich exceeds several keV, or even several hundred keV. Thus, thecollection efficiency of X-ray or gamma photons is low. Collectionefficiency corresponds to a number of photons reaching the detectornormalized by the number of photons emitted. Another difficulty is thelow detection efficiency of detectors sensitive at such energies, inparticular when recourse to compact detectors is privileged. Theconsequence of the low collection efficiency, combined with a lowdetection efficiency, is that it is often necessary to acquire an imagewith a relatively long acquisition time, so as to increase the number ofphotons detected. It is thus common to keep a gamma camera stationaryfor several or even several tens of seconds, so as to obtain asufficient measurement statistic.

Certain developments have been made, allowing a three-dimensionalreconstruction of the position of irradiating sources to be generated.It is a question of applying the principles of triangulation, and oftaking a plurality of images of the same irradiating sources, whilemoving the gamma camera between the acquisition of the various images.However, such a method requires high computing powers. In addition, thismethod does not allow a movement of irradiating source is in astationary environment to be taken into account.

The inventors have suggested a method that is simple, inexpensive interms of memory size and of computing power, and that allows a movementof a gamma camera with respect to stationery irradiating sources, or amovement of the irradiating sources with respect to a gamma camera, tobe taken into account. The method allows gamma images to besatisfactorily acquired while moving a gamma camera relative to anirradiating source or relative to a plurality of irradiating sources.

SUMMARY OF THE INVENTION

A first subject of the invention is a method for forming areconstruction image, the reconstruction image showing a position ofirradiating sources in an observation field, the method employing adevice comprising a gamma camera joined to a visible camera, the devicebeing such that:

-   -   the gamma camera and the visible camera define the observation        field;    -   the visible camera is configured to form a visible image of the        observation field;    -   the gamma camera comprises pixels, which are configured to        detect ionizing electromagnetic radiation generated by an        irradiating source potentially present in the observation field,        the pixels lying in a detection plane;    -   the gamma camera is configured to form a gamma image, allowing a        position, in the field of observation, of irradiating sources,        the radiation of which is detected by the pixels, to be        estimated;        the method comprising the following iterative steps:    -   a) at an initial measurement time, acquiring an initial gamma        image with the gamma camera and acquiring an initial visible        image with the visible camera;    -   b) on the basis of the initial gamma image and of the initial        visible image, initializing the reconstruction image;    -   c) acquiring a gamma image with the gamma camera and acquiring a        visible image with the visible camera, at a measurement time;    -   d) comparing the visible image at the measurement time with a        visible image at a prior time, the prior time being the initial        measurement time or a preceding measurement time, then,        depending on the comparison, estimating a field of movement        between the visible images acquired at the prior time and at the        measurement time, respectively;        -   updating the reconstruction image, depending:        -   on a reconstruction image established at the prior time;        -   on the movement field resulting from step d);    -   e) and on the gamma image acquired at the measurement time;    -   f) optionally superposing the reconstruction image, or a portion        of the reconstruction image, and the visible image;    -   g) reiterating steps c) to f) while incrementing the measurement        time, until the iterations are stopped;

the device being moved between at least two measurement times, such thatone observation field corresponds to each measurement time.

Thus, the reconstruction image shows a position of the irradiatingsources in the observation field. According to one embodiment, thereconstruction image, at a measurement time, is defined only in theobservation field corresponding to the measurement time.

Preferably, between at least two measurement times, the device is moved,so that the observation fields before and after the movementrespectively comprise a common portion.

The gamma camera defines a gamma observation field. The visible cameradefines a visible observation field. The observation field of the devicecorresponds to an intersection of the gamma observation field and of thevisible observation field.

A gamma image allows gamma sources potentially present in the gammaobservation field to be located.

According to one preferred embodiment, at each measurement time, eachpixel having detected radiation generated by an irradiating source isassociated with a position in an object surface lying facing thedetection plane, the reconstruction image corresponding to a spatialdistribution of the irradiating sources inside the observation field atthe measurement time over the object surface. The object surface mayadvantageously be an object plane, lying parallel to the detectionplane, each position being coplanar.

According to this embodiment, the reconstruction image, at themeasurement time, is defined only in an intersection between theobservation field at the measurement time and the object surface. Thereconstruction image, at the measurement time, may be defined only in anintersection between the observation field at the measurement time andthe object surface.

Step e) may comprise:

-   -   i) estimating a reconstruction image, at the measurement time,        on the basis of the reconstruction image established at the        prior time and of the movement field obtained in step d);    -   ii) on the basis of the estimated reconstruction image,        estimating the gamma image at the measurement time;    -   iii) comparing the gamma image estimated at the measurement time        and the gamma image acquired at the measurement time;    -   iv) depending on the comparison, updating the reconstruction        image at the measurement time.

The method may comprise determining a registration functionrepresentative of a spatial offset between the gamma camera and thevisible camera, the registration function being used in step e). Theregistration function allows a movement field of the gamma image to bedefined on the basis of the movement field resulting from step d). Thisallows a movement of two gamma images, respectively acquired at theprior time and at the measurement time, to be obtained on the basis ofthe movement established for two visible images at said times.

Step d) may comprise selecting noteworthy points in the visible imagesacquired at the prior time and at the measurement time, respectively,the movement field comprising a two-dimensional movement of the selectednoteworthy points.

The noteworthy points may preferably be considered to belong to the samesurface, or even to the same plane. It may notably be a question of theobject surface or of the object plane or of a plane parallel to theobject plane.

The method may be such that step d) comprises, at each measurement time:

-   -   extracting first noteworthy points from the visible image        acquired at the measurement time;    -   extracting second noteworthy points from the visible image        acquired at the prior time;    -   matching first noteworthy points and second noteworthy points,        so as to form pairs of matched points, each pair being formed        from a first noteworthy point and from a second noteworthy        point;    -   for each pair of matched points, determining a movement;    -   obtaining the movement field on the basis of the movements        determined for each pair of matched points.

The method may comprise:

-   -   generating a first mesh of the observation field, at the        measurement time, using first noteworthy points, the latter        forming first vertices of the first mesh;    -   generating a second mesh of the observation field, at the prior        time, using second noteworthy points, the latter forming second        vertices of the second mesh;    -   depending on the movements determined for various pairs of        noteworthy points, determining, by interpolation, movements at        points belonging to the first mesh and to the second mesh;    -   determining the movement field on the basis of the movement of        each matched vertex and of the interpolated movements.

According to one embodiment:

-   -   the device is coupled to a movement sensor, so as to estimate a        movement of the device between two successive times;    -   step d) takes into account the estimated movement to estimate or        validate the movement field.

One aspect of the invention is that the movement field, establishedusing the visible images, is considered to be representative of themovement of the irradiating sources between two measurement times, theregistration function making up for any difference.

According to one embodiment, the gamma camera is configured tosimultaneously acquire, in steps a) and c), gamma images in variousenergy bands, steps b) and d) to f) being implemented for each energyband respectively, so as to obtain, at each measurement time, areconstruction image in various energy bands.

According to one embodiment, the gamma camera is configured tosimultaneously acquire, in steps a) and c), gamma images in variousenergy bands, the method comprising a linear combination of variousgamma images respectively acquired, at the same measurement time, invarious energy bands, so as to obtain a combined gamma image, steps b)and d) to f) being carried out on the basis of a combined gamma imageformed at each measurement time. The combined image may be formed by aweighted sum of gamma images acquired at various energies, the weightedsum using weighting factors, the weighting factors being determined onthe basis of an emission spectrum of a radioactive isotope, such thateach reconstruction image is representative of a spatial distribution ofthe activity of the isotope in the observation field corresponding tothe measurement time.

Another subject of the invention is a device, comprising:

-   -   a gamma camera, comprising pixels, defining a gamma observation        field, the gamma camera being configured to determine a position        of sources of X-ray or gamma irradiation in the gamma        observation field;    -   a visible camera, allowing a visible image of a visible        observation field to be formed;    -   the intersection of the gamma observation field and of the        visible observation field being nonzero, and defining an        observation field of the device;    -   an image-processing unit, configured to receive, at various        measurement times:        -   a gamma image, formed by the gamma camera;        -   a visible image formed by the visible camera;            the processing unit being configured to carry out steps b)            and d) to g) of a method according to the first subject of            the invention.

The invention will be better understood on reading the description ofexemplary embodiments that are described, in the rest of thedescription, with reference to the figures listed below.

FIGURES

FIG. 1A schematically shows a measuring device placed facing structuralelements and irradiating sources.

FIGS. 1B and 1C show gamma images and visible images acquired at onemeasurement time, respectively.

FIG. 2A schematically shows a measuring device placed facing structuralelements and irradiating sources, at another measurement time.

FIGS. 2B and 2C show gamma images and visible images acquired at theother measurement time, respectively.

FIG. 3 schematically shows the object plane with respect to the gammacamera.

FIG. 4 shows the main steps of a method for obtaining a reconstructionimage.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIG. 1A shows a measuring device 1 allowing the invention to beimplemented. The measuring device comprises a gamma imager 2, or gammacamera. The gamma imager is configured to detect ionizingelectromagnetic rays, of X-ray or gamma-ray type, the energy of which isgenerally comprised between 10 keV and 10 MeV. The gamma imagercomprises pixels, each pixel corresponding to an elementary spatialregion of the observation field. The pixels lie in a detection plane P.When the elementary spatial region corresponding to a pixel comprises anemitting source of X-ray or gamma radiation, some of the radiationemitted by the source reaches the pixel and is detected by the latter.Thus, the amplitude of the signal of a pixel of the gamma imageincreases as the elementary spatial region with which it is associatedbecomes more irradiant, i.e. emits more X-ray or gamma radiation. In therest of the description, the examples are given in relation to sourcesof gamma irradiation, this corresponding to the most frequent case ofapplication. It is directly transposable to sources of X-rayirradiation.

The gamma imager may be a Compton gamma camera, a pinhole-collimatorgamma camera or coded-aperture gamma camera. It may also be a question,non-exhaustively, of a gamma camera the collimator of which comprisesparallel channels, or convergent channels, or divergent channels. Thus,the term gamma camera corresponds to an imager having an observationfield and configured to form an image allowing irradiation sources to belocated in the irradiation field. Whatever the type of gamma imager, itallows a gamma image comprising pixels, each pixel corresponding to oneelementary spatial region of the observed environment, to be formed.Certain gamma imagers have a spectrometric function, in the sense thatthey allow radiation detected in various spectral bands to be spectrallyseparated. When this type of imager is used, it is possible to formvarious gamma images of a given observation field, each imagecorresponding to one spectral band. The image may likewise beestablished by considering a combination of spectral bands, whichcorrespond to the emission spectrum of an isotope. The combination maybe a weighted sum. The image is then representative of a spatialdistribution of the activity of the isotope in question.

With certain gamma imagers, in particular Compton Gamma cameras orcoded-aperture gamma cameras, the image acquired by the imager does notallow the emitting sources in the observation field to be vieweddirectly. The acquired image undergoes processing, taking into account aresponse function of the camera, so as to allow a gamma image in whichthe brightness of each pixel corresponds to the emission intensity ofthat elementary spatial region of the observation field which isassociated with the pixel to be obtained.

It is conventional for gamma cameras to be associated with visiblecameras. These are standard cameras, allowing a visible image of theobservation field to be formed. The device shown in FIG. 1A comprises avisible camera 3, which is securely fastened to the gamma camera 2. Thevisible camera 3 and the gamma camera 2 each have an observation field,extending about their respective optical axes. In FIG. 1A, the opticalaxes and observation fields of the gamma camera 2 and of the visiblecamera 3 are denoted Ω₂, Δ₂, Ω₃ and Δ₃. Generally, the visibleobservation field is larger than the gamma observation field. Thevisible and gamma observation fields overlap: their intersection isnonzero. The latter is as large as possible.

The visible camera 3 is usually placed at a small distance from thegamma camera, such that their respective optical axes are closetogether, and preferably parallel to each other. Thus, beyond a certaindistance, generally smaller than 1 m, or even than 50 cm, theobservation field of the gamma camera is included in the observationfield of the visible camera. The intersection of the two observationfields forms the observation field Ω of the device.

Preferably, the optical axes of the gamma camera 2 and of the visiblecamera 3 are aligned. The gamma camera and the visible camera arecalibrated to correct a parallax error due to the offset of the twooptical axes, and to take into account geometric distortions due to thelenses of the visible camera, in particular at the field edge. Thecalibration also takes into account a difference in the number and sizeof pixels between the visible camera and gamma camera. The calibrationallows a registration function g to be defined.

A processing unit 4 receives the images generated by the gamma camera 2and the visible camera 3. The image-processing unit is notablyconfigured to merge the images obtained from the visible camera 3 andthe images obtained from the gamma camera 2, notably by taking intoaccount the registration function. The objective is to obtain acomposite image corresponding to the visible image, but on which theirradiating sources detected by the gamma camera appear. The processingunit 4 is connected to a memory in which image-processing instructionsare coded. The processing unit 4 comprises a processor, a microprocessorfor example, configured to implement instructions corresponding to thesteps described below.

As mentioned in relation to the prior art, the acquisition time of agamma image is generally several seconds, or even several tens ofseconds. This is due to the low collection efficiency combined with thelow detection efficiency.

FIG. 1A shows irradiating sources 10 _(a), 10 _(b) and 10 _(c), whichare emitting sources of γ radiation, and structural elements 11 _(a), 11_(b) and 11 _(c) schematically representing the visible environmentaround the irradiating sources. The device 1 occupies a position and hasan orientation in the observed environment. FIG. 1B schematically showsa gamma image obtained by the gamma camera. In said image, by way ofillustration, the irradiating sources 10 a and 10 _(b) present in theobservation field have been represented in the form of circles. FIG. 1Cschematically shows a visible image obtained with the visible camera,and allowing objects present in the observation field to be observed.The term object corresponds to structural elements present in theobservation field. It may be a question of portions of architecturalelements (walls, doors, windows, ridges) and/or of portions ofindustrial installations (pipes, specific pieces of equipment) and/or ofitems of furniture (table, chair, chest, various items).

FIG. 2A shows the device, in the same environment, in a differentposition and with a different orientation to those shown in FIG. 1A.Generally, because of the sensitivity of the gamma camera, severalseconds, or even minutes, of acquisition are required to obtain arepresentation of each irradiating source with a sufficientsignal-to-noise ratio. During the acquisition of a gamma image, thegamma camera must preferably be held stationary.

FIG. 2B shows an obtained gamma image. The circles drawn with solidlines show the irradiating sources 10 _(a), 10 _(b) and 10 _(c). In thisimage, the dashed lines show the positions of the irradiating sources 10a and 10 _(b) such as they appear in FIG. 2B. In the gamma image, thechange in orientation of the gamma camera between FIGS. 2A and 2B hasresulted in a movement of the irradiating sources, this movement beingillustrated by two arrows. It is possible to take into account anenvironment, in which each irradiating source detected by the gammacamera has a fixed position, represented by two-dimensional coordinatesin an object space. When the gamma camera is moved in the object space,the gamma sources present in the observation field are detected andproduce an exploitable signal in the gamma image. Generally, the signalformed in the gamma image has a relatively low signal-to-noise ratio,the latter depending on the level of irradiation produced by the sourceon the gamma camera. In order to improve the signal-to-noise ratio, thesignal produced by an irradiating source on a gamma camera may beaccumulated, for example via a moving average. Thus, each time a gammasource is present in the observation field, the signal that it generatesin the gamma image is taken into account, so as to improve the detectionstatistic associated with the source. The gamma image is then integratedinto a reconstruction method, taking into account gamma images acquiredbeforehand and/or subsequently.

A visible image acquired by the visible camera is associated with eachacquisition of a gamma image. The visible camera 3 acquires a visibleimage of the observed scene. The observation field Ω of the imagingdevice, i.e. at the intersection of the respective observation fields ofthe gamma camera and of the visible camera, defines an object space. Theobject space contains points (x, y), each point being associated withone pixel (u, v) of the gamma image. The pixels belong to a detectionplane P, in which the gamma image is formed. The points (x, y) of theobservation field have two-dimensional coordinates, and correspond topixels of the visible image, after the registration function has beentaken into account.

An important element of the invention is that the points of the objectframe of reference are considered to belong to the same objectprojection plane P_(o). The angular observation field Ω₂ of the gammacamera, which extends about the optical axis Δ₂, describes a segment ofa sphere S (see FIG. 3). The object plane P_(o) corresponds to a plane,tangent to the sphere S, and perpendicular to the optical axis Δ₂. Theirradiating sources present in the observation field are considered tobe coplanar and to belong to the object plane. The distance between thedetection plane and the object plane is an arbitrary distance, which maynot be known. It is not taken into account in the reconstruction methoddescribed below. According to one variant, the irradiating sources areconsidered to belong to the same three-dimensional surface, lying facingthe detection plane P.

After image processing, allowing an image merger to be performed(thresholding and/or superposition with one image visible through theother), and taking into account the registration function, theirradiation levels associated with the points in the object frame ofreference, and that are considered to be significant, are superposed onthe visible image V_(k) in the form of a colour code.

Thus, the observation field Ω of the device bounds a portion of theobject plane, in which portion the reconstruction is performed.

The correspondence between each point (x, y) of the object plane P_(O)and each pixel (u, v) of the gamma image depends on a spatial responsefunction of the gamma camera. When gamma photons are emitted from apoint (x, y) of the object frame of reference, toward the gamma camera,the trace that they form in the gamma image results from a spatialresponse function F of the camera. When a gamma camera based on apinhole collimator is employed, the spatial response function takes intoaccount the aperture of the pinhole. It may be approximated by an imageof an irradiating source centred in the field of observation, which isused by way of impulse response. When a coded-aperture gamma camera isemployed, the response function takes into account the geometry of themask. When a Compton gamma camera is employed, the response functiondepends on the detected energy and on the position of pixels havingdetected radiation, in the detection plane P. Thus, in a Compton gammacamera, the response function may vary at each measurement time.

The correspondence between a point in the observation field and a pixelof the gamma image may for example be determined via a convolutionproduct of the gamma image and the response function (notably in thecase of a gamma camera equipped with a pinhole collimator), or via arectilinear projection (notably in the case of a gamma camera equippedwith a coded-aperture collimator). The projection may also be of anothertype, for example, and nonlimitingly, stereographic or orthographic.

Thus, the response function F makes it possible to pass from thedetection plane, in which the gamma image is acquired, to the objectplane P_(O), which corresponds to the observed scene, and in which theposition of irradiating sources is sought. In the rest of thedescription, the notation F corresponds to a projection of an imageformed in the object plane toward the detection plane. The notation F⁻corresponds to a retroprojection of an image formed in the detectionplane P toward the object plane P_(o). In a first approach, F is alinear operator and F⁻ corresponds to a transpose of F.

If M_(k) is a gamma image acquired at a time t_(k), and I_(k) is animage showing a reconstruction of the irradiation sources in the fieldΩ_(k) observed at the time t_(k), the following is obtained:

I _(k)(x, y)=F ⁻[M _(k)(u, v)]  (1)

where:

-   (x, y) are coordinates, in the object plane, parallel to the    detection plane, and corresponding to coordinates (u, v) in the    detection plane;-   I_(k)(x, y) corresponds to an image of the distribution of    irradiating sources in the observation field. It is a question of a    two-dimensional matrix each term of which represents an emission    rate, in a spectral band, or in a plurality of combined spectral    bands, of an irradiating source in the object plane, at coordinates    (x, y), at the measurement time t_(k). As described above, when    there are a plurality of spectral bands in the emission spectrum of    an isotope, the image I_(k) is representative of a spatial    distribution of the activity of the isotope. Generally, I_(k)    corresponds to an estimation of the distribution of the irradiation    after at least one gamma image acquired by the gamma camera has been    taken into account. Each term of the image I_(k)(x, y) corresponds    to an estimation of an emission rate of gamma (or X-ray) photons, or    of an activity, at point (x, y) in the object plane. The size of the    image I_(k) is Nx, Ny, where Nx and Ny designate the number of    pixels of the gamma camera along a horizontal axis and a vertical    axis, respectively. The size of the matrix I_(k) is identical to    that of a gamma image acquired by the gamma camera.

F⁻ is a retroprojection operator, allowing passage to the object planeP_(o) from the detection plane P.

Representing the position of the irradiating sources in atwo-dimensional spatial distribution (or map) allows the reconstructionimage to be superposed on a visible image acquired by the visiblecamera, simply.

This method, the main steps of which are schematically shown in FIG. 4,allows gamma sources to be located in an environment, while allowing arelative movement of the measuring device 1 with respect to theirradiating sources. Generally, the device 1 is moved with respect tothe environment and, during the movement, a plurality of gamma imagesare acquired. Between two successive acquisitions, the relative movementof the object frame of reference with respect to the device 1 isrecorded. The objective is to allow, in the object plane, the positionof irradiating sources moving with respect to the measuring device 1 tobe tracked. The relative movement may be a movement of the measuringdevice with respect to an irradiating source, the latter remainingstationary in the environment. It may also be a question of a movementof the irradiating source with respect to the measuring device, thelatter remaining stationary in the environment.

Step 100: Initialization

For a first acquisition, the measuring device 1 is placed at a firstposition, at a first time t₁. A gamma image M₁ is acquired. Generally,the acquisition time of each gamma image is comprised between 1 ms and 5s, and preferably comprised between 50 ms and 500 ms, and is for example100 ms. In the gamma image, each irradiating source present in the fieldtakes the form of a trace, the brightness of which depends on theirradiation, generated by the irradiating source, and detected by thegamma camera.

When the gamma camera is able to perform a spectrometric function, agamma image is acquired in a given energy spectral band ΔE, or in acombination of energy bands, as described above.

A visible image V₁ is acquired at the first time t₁, or at a timeconsidered to be consecutive to the first time t₁. The main objectivesof the initial visible image V₁ is to obtain noteworthy points in theobserved visible scene. The noteworthy points, and the use made thereof,are described in more detail in relation to step 130.

The gamma image M₁ is divided by an estimation image {circumflex over(M)}₁. The estimation image {circumflex over (M)}₁ corresponds to anestimation of the gamma image M₁. For the first acquisition, theestimation image {circumflex over (M)}₁ is a predetermined image. It isfor example uniform and made up of 1.

The ratio

$U_{1} = \frac{M_{1}}{{\overset{\Cap}{M}}_{1}}$

corresponds to an error term in the measurement with respect to theestimation. The ratio U₁ is computed term-by-term, for each pixel (u, v)of the gamma image M₁ and of its estimate {circumflex over (M)}₁.

Step 110: Back-Propagation of the Error and Update of the Reconstruction

The error term U₁ is propagated to the object plane, so as to update areconstruction image I₁ such that:

W ₁(x, y)=F ⁻[U ₁(u, v)]  (2)

and

I ₁(x, y)=I ₀(x, y)×W ₁(x, y)   (3)

The image I₀(x, y) is a predetermined initialization image that forexample contains only real positive numbers, only 1 s for example.

Following steps 100 to 110, steps 120 to 190 are carried outiteratively. To each iteration corresponds an iteration rank k. k is aninteger comprised between 2 and K. K corresponds to the iteration rankwhen the iterations are stopped.

Step 120: Acquisition of a Gamma Image M_(k) and of a Visible ImageV_(k) at a Time t_(k).

A visible image V_(k) is acquired at a time t_(k). A gamma image M_(k)is also acquired at the time t_(k), or at an acquisition time consideredto be simultaneous with the time t_(k). Preferably, the acquisitiontimes M_(k) of each gamma image are identical in each iteration k.

Step 130: Estimation of a Movement Field D_(k)

The objective of this step is to form a movement field D_(k)representative of a two-dimensional movement of the visible imageV_(k−1) with respect to the visible image V_(k). The image V_(k−1) is avisible image acquired in a preceding iteration or, when k=2, in step100. The movement field D_(k) comprises, at various coordinates (x, y)in the object plane, a movement vector d_(k), corresponding to amovement along the X-axis and a movement along the Y-axis. Each movementvector is a vector of size equal to 2. Thus, only a two-dimensionalmovement, in a plane parallel to the object plane P_(O), or coincidentwith the latter, is taken into account.

The device 1 may have been moved or reoriented between the acquisitionsof the images V_(k−1) and V_(k). Alternatively, certain elements of theobject space may have been moved between the two acquisitions. This isnotably the case when an irradiating source moves in the object frame ofreference.

In each image V_(k−1) and V_(k), noteworthy points are identified. Thenoteworthy points are points that are easily identifiable viaconventional image processing. It is for example a question of pointsforming an outline or edge of an object, or of points that are ofparticularly high contrast from the point of view of brightness or ofcolour. Thus, a noteworthy point may be a point corresponding to a highLaplacian or brightness gradient.

The number of noteworthy points detected in each image V_(k−1) and V_(k)is preferably comprised between a few tens and a few hundred or even ismore than 1000. The noteworthy points detected in the two images V_(k−1)and V_(k) form a set E_(k−1) and a set E_(k), respectively.

The noteworthy points in an image may be detected by implementing analgorithm, for example a Harris corner detector. Following theirdetection, the noteworthy points are characterized, so as to allow theirpotential identification in the two images V_(k−1) and V_(k). Thecharacterization aims to characterize each noteworthy point and itsenvironment. This may be achieved using feature-description algorithmsknown to those skilled in the art, such as DAISY or LUCID or FREAK. Witheach noteworthy point is associated a descriptor vector, allowing it tobe recognized in the two images V_(k−1) and V_(k).

On the basis of their description and of their characterization, thenoteworthy points identified in the images V_(k−1) and V_(k) arematched, so as to establish pairs of noteworthy points, each pairassociating a noteworthy point of the image V_(k−1) with a noteworthypoint of the image V_(k), the matched noteworthy points havingdescriptor vectors that are considered to be identical. The matchednoteworthy points correspond to the same point of the observed scene,this point appearing in both visible images V_(k−1) and V_(k).

Preferably, the noteworthy points of each image are considered to becoplanar: they lie in a plane parallel to the object plane.

On the basis of the matched noteworthy points, the images V_(k−1) andV_(k) are meshed, on the basis of the sets E_(k−1) and E_(k),respectively. Delaunay mesh generation may be used, this type of meshgeneration defining triangular mesh cells, each mesh cell lying betweenthree noteworthy points that are different from one another and matchedin the two images V_(k−1) and V_(k). For each vertex of the mesh,present in both images V_(k−1) and V_(k), a two-dimensional movementd_(k) is estimated. For an optimal implementation of the invention, itis preferable for the movement of the device, or of the elements formingthe object frame of reference, to be relatively slow, so as to maximizethe number of vertices of the mesh present in both images V_(k−1) andV_(k). A sufficiently slow and fluid movement and a sufficient lightlevel also prevents blurring of the visible image. Preferably, theobservation fields before and after a movement overlap by at least 50%,or even at least 80%, or 90%.

An estimation of a movement vector d_(k) is thus obtained for eachvertex of the mesh present in both images V_(k−1) and V_(k). Since themovement vector is defined only at the vertices present in the imagesV_(k−1) and V_(k), an interpolation, for example a linear interpolation,is performed, so as to obtain an estimation of a movement d_(k) at eachpoint (x, y) of the observation field Ω_(k) of the device, at leastwithin the mesh. More precisely, the movement is determined, at least ina mesh established in the intersection of the observation fields Ω_(k−1)and Ω_(k) corresponding to the visible images V_(k−1) and V_(k) acquiredat the measurement times k−1 and k, respectively. The movement fieldD_(k) is then formed, each term of which corresponds to the movementvector d_(k)(x, y) determined at least at each point (x, y) included inthe mesh.

The movement field D_(k), outside the mesh, may be estimated byextrapolation, on the basis of the movement vectors d_(k)(x, y)established inside the mesh. This allows a movement field vector D_(k)to be obtained for the entire observation field of the image. In oneembodiment, the device comprises a movement sensor 5, allowing anangular movement of the device between the acquisitions of the visibleimages V_(k−1) and V_(k) to be estimated. The movement sensor may be amagneto-inertial measurement unit, or comprise at least one gyrometer.In this case, the movement field, outside the mesh, may be estimated bycombining the movement field obtained inside the mesh with the movementof the device between the two images.

Other methods allow a movement field D_(k) between two successive imagesto be estimated. It may for example be a question of optical-flowmethods, which allow a small movement between two successive images tobe estimated.

The movement field D_(k) is at least partially determined by observing amovement of objects present in the visible images V_(k−1) and V_(k).According to a first possibility, the objects remain stationary in theobserved scene: the movement field is established by observing amovement of objects, or portions of objects, present in both imagesV_(k−1) and V_(k). In this case, the movement field corresponds to themovement of the camera with respect to the objects present in theobservation field of said camera. According to a second possibility, theobjects move even though the device has not been moved between theacquisition of the images V_(k−1) and V_(k). In this case, the movementfield D_(k) corresponds to the movement of the objects relative to thevisible camera. More generally, the movement field D_(k) corresponds tothe relative movement of objects, present in the observation field ofthe visible camera, relative to the latter.

When it is not possible to determine the movement field outside of themesh, by extrapolation or using movement measurements, the movementfield outside the mesh is considered to be uniform, and of constantvalue—a zero vector may for example be used.

At the end of this step, a movement field D_(k) of (2, Nx, Ny) size isobtained. At each point (x, y) in question, a field D_(k) ^((X)) ofmovement along the X-axis, of (Nx, Ny) size, and a field D_(k) ^((Y)) ofmovement along the Y-axis, of (Nx, Ny) size, are established.

Step 140: Estimation of the Reconstruction Image Î_(k)

On the basis of the reconstruction image I_(k−1)(x, y) resulting from apreceding iteration, or resulting from the initialization, and knowingthe movement field D_(k), a reconstruction image Î_(k) is estimated,taking into account the registration function. This step is an importantelement of the invention.

This step is based on the assumption that the movement of the positionof the irradiating sources, in the reconstruction image I_(k), betweenthe times k−1 and k, may be obtained by considering the movement fieldD_(k) measured between two successive visible images associated with thetimes k−1 and k, respectively, the visible images V_(k−1) and V_(k)being associated with the gamma images M_(k−1) and M_(k), respectively.

Thus: Î _(k)=(B*I _(K−1))^(1−α) ×I _(k−1) ·g(D _(k))^(α)  (4)

where:

-   × is a term-by-term multiplication operator (Hadamard product);-   I_(k−1) corresponds to the reconstruction image resulting from a    preceding iteration. It is a question of a matrix, the same size as    a gamma image M_(k), and each point of which corresponds to an    emission rate, in a given spectral band or in a combination of    spectral bands, or to an activity of an isotope.

B is a filter allowing the image I_(k−1) to be smoothed. It may be alow-pass filter, a Gaussian filter for example. The convolution of theimage I_(k−1) and the smoothing filter B allows the reconstruction imageI_(k−1) to be blurred. Recourse to such a filter allows an a priori onthe position of the irradiating sources such as described by thereconstruction image I_(k−1) to be established. According to onealternative, the filtering may be performed according to the expression:

Î _(k) =B*[I _(k−1) ^(1−α) ×I _(k−1) ·g(D _(k))^(α)]  (4′)

g corresponds to the registration function; g(D_(k)) is a matrixfunction taking into account movement in the object plane, while takinginto account the registration function g of the visible and gammacameras.

I_(k−1)·g(D_(k)) is the reconstruction image I_(k−1) after applicationof the registered movement vector.

At each point (x, y) in the observation field, D_(k) allows a movementD_(k) ^((X))(x, y) along the X-axis and a movement D_(k) ^((Y))(x, y)along the Y-axis to be defined. The matrix function g(D_(k)) allows amovement, in the object plane, to be established depending on themovement D_(k). It allows the coordinates (x_(k), y_(k)) at the time kto be estimated depending on coordinates (x_(k−1), y_(k−1)) at the timek−1, such that:

x _(k) =x _(k−1) +g _(X)[D _(k) ^((X))(x, y)]  (5.1)

y _(k) =y _(k−1) +g _(Y)[D _(k) ^((Y))(x, y)]  (5.2)

where g_(X) and g_(Y) correspond to components of the registrationfunction established along the X-axis and along the Y-axis,respectively.

Thus, g(D_(k)) is a matrix function allowing passage between (x_(k−1),y_(k−1)) and (x_(k), y_(k)), such as explained by expressions (5.1) and(5.2). It is a question of a change of variables.

The exponent α is strictly positive and strictly lower than 1: 0<α≤1.The exponent α allows a memory effect to be achieved: the closer α getsto 1, the more the memory of preceding gamma-image acquisitions ispreserved.

The movement field D_(k) is established by observing a movement ofobjects between the visible images V_(k−1) and V_(k), in the observationfield of the visible camera. Thus, the movement of the irradiatingsources, in the two reconstruction images I_(k−1) and Î_(k) is based onthe detection of objects, forming noteworthy points in the visibleimages V_(k−1) and V_(k), matching thereof, and the computation of therespective movements thereof between the two visible images. Themovement thereof allows a field D_(k) of movement in the visible imageto be established, which is assigned to the gamma image, afterapplication of the registration function.

Thus, to within the registration function, movement of the irradiationsources is considered to be able to be determined from the movementfield computed on the basis of the visible images. An important point inthe invention is that the movements of objects observed in the visibleimage are considered to be representative of the movements of the gammasources, the latter being shown in the reconstruction image. Thissignificantly limits the complexity of the computations and theresources required to perform the computations. The method does notrequire complex techniques, such as triangulation, to be implemented ora three-dimensional position of the irradiating sources in theenvironment to be estimated.

Step 150: Estimation of the Gamma Image {circumflex over (M)}_(k) at theTime k

The estimated reconstruction image Î_(k), obtained in step 140, isprojected into the detection plane, so as to estimate the gamma image{circumflex over (M)}_(k) corresponding to iteration k. The gamma image{circumflex over (M)}_(k) is estimated using the expression

{circumflex over (M)}_(k)(u, v)=F[Î _(k)(x, y)]  (6)

where F corresponds to a projection operator describing the projectionbetween the object plane P_(o) and the detection plane P.

Step 160: Determination of an Error Term

In this step, on the basis of the image M_(k) acquired at the time k,and resulting from step 120, and of the estimation {circumflex over(M)}_(k) resulting from step 140, the measurement error is computed:

$\begin{matrix}{U_{k} = {\frac{M_{k}}{{\overset{\Cap}{M}}_{k}}.}} & (7)\end{matrix}$

Step 170: Back-Propagation of the Error

The error term U_(k) is back-propagated to the object plane using theexpression:

W _(k)(x, y)=F ⁻[U _(k)(u, v)]  (8)

Step 180: Update of the Reconstruction Image

In this step, the reconstruction image, corresponding to the time k, isupdated according to the expression,

I _(k) =I _(k−1) ×W _(k)   (9)

in which × designates a term-by-term multiplication.

According to one embodiment, a non-linear function h is taken intoaccount, such that

I _(k) =I _(k−1) ×h(W _(k))   (10)

Thus, at each measurement time k, the reconstruction image I_(k) isupdated by taking into account the gamma image M_(k) acquired at themeasurement time. The gamma image M_(k) is taken into account via theback-propagated error W_(k).

The reconstruction image I_(k) is defined in an intersection between theobject plane P_(O) and the observation field Ω_(k) at the measurementtime k. Preferably, the reconstruction image I_(k) is not definedoutside of the observation field Ω_(k) at the measurement time k. Thisallows the reconstruction process to be simplified, by not taking intoaccount irradiation sources potentially present outside of theobservation field Ω_(k). The reconstruction image then contains only thepositions of the irradiating sources present in the observation field atthe measurement time k, obtained on the basis of gamma images M_(k)acquired at the measurement time or of gamma images acquired at priortimes, preceding the measurement time. On each movement of the measuringdevice 1, the spatial extent of the reconstruction image is modified, soas to limit it to the intersection between the object plane P_(O) andthe observation field Ω_(k) at the measurement time k.

The reconstruction image may be of a size smaller than or equal to thesize of each gamma image.

Step 180: Superposition of the Images.

The reconstruction image I_(k) is given a certain level of transparency,then is superposed on the visible image V_(k), the superposition beingcarried out without taking into account the registration function. Thisallows a correspondence to be achieved between the objects observed inthe visible image and the irradiating sources shown in thereconstruction image.

Step 190:

Reiteration of steps 130 to 190, while incrementing the iteration indexk.

According to one embodiment, the device comprises a movement sensor 5,for example an inertial measurement unit. The movement sensor comprisesa gyrometer, optionally complemented by an accelerometer and/or amagnetometer. Generally, the movement sensor allows an angular movementbetween two successive acquisition times to be obtained. Informationobtained from the inertial measurement unit may be taken into account instep 130, so as to limit the risk of error in the matching of thenoteworthy points. It may also be used to complete the movement fieldD_(k) outside of the mesh formed between the matched noteworthy points.The translational movement of the camera and the positioning of theobjects may also be estimated using other devices such as LIDAR, GPSpositioning systems, fixed radiofrequency beacons or an inertialnavigation system.

According to one embodiment, the gamma camera is able to perform aspectrometric function. Steps 100 to 180 are carried out simultaneously,in each energy band ΔE. It is then possible to obtain as manyreconstruction images as there are considered energy bands.

Steps 100 to 180 may also be carried out on the basis of gamma imagesthat are respectively acquired in various energy bands, then combined,as described above, so as to take into account an emission spectrum of aradioactive isotope selected beforehand. In this case, thereconstruction image may be likened to a spatial distribution of theactivity of the radioactive isotope, in the observation field.

The invention is applicable to various nuclear installations, or, moregenerally, to operations of seeking for and characterizing radioactivesources.

1. A method for forming a reconstruction image, the reconstruction imageshowing a position of irradiating sources in an observation field, themethod using a device comprising a gamma camera joined to a visiblecamera, wherein: the gamma camera and the visible camera define theobservation field; the visible camera is configured to form a visibleimage of the observation field; the gamma camera comprises pixels, whichare configured to detect ionizing electromagnetic radiation generated byan irradiating source potentially present in the observation field, thepixels lying in a detection plane; the gamma camera is configured toform a gamma image, allowing a position, in the field of observation, ofirradiating sources, the radiation of which is detected by the pixels,to be estimated; the method comprising the following iterative steps: a)at an initial measurement time, acquiring an initial gamma image withthe gamma camera and acquiring an initial visible image with the visiblecamera; b) on the basis of the initial gamma image and of the initialvisible image, initializing the reconstruction image; c) acquiring agamma image with the gamma camera and acquiring a visible image with thevisible camera, at a measurement time; d) comparing the visible image atthe measurement time with a visible image at a prior time, the priortime being the initial measurement time or a preceding measurement time,then, depending on the comparison, estimating a field of movementbetween the visible images acquired at the prior time and themeasurement time, respectively; e) updating the reconstruction image,using: a reconstruction image established at the prior time; themovement field resulting from step d); and the gamma image acquired atthe measurement time; f) optionally superposing the reconstructionimage, or a portion of the reconstruction image, and the visible image;g) reiterating steps c) to f) while incrementing the measurement time,until the iterations are stopped; wherein the method includes moving thedevice between at least two measurement times, such that one observationfield corresponds to each measurement time.
 2. The method according toclaim 1, wherein at each measurement time, each pixel having detectedradiation generated by an irradiating source is associated with aposition on an object surface lying facing the detection plane; thereconstruction image corresponds to a spatial distribution of theirradiating sources inside the observation field at the measurement timeover the object surface.
 3. The method according to claim 2, wherein theobject surface is an object plane lying parallel to the detection plane,each position within the object surface being coplanar.
 4. The methodaccording to claim 1, wherein the reconstruction image, at themeasurement time, is defined only in an intersection between theobservation field at the measurement time and the object surface.
 5. Themethod according to claim 1, wherein at each measurement time, step e)comprises i) estimating a reconstruction image, at the measurement time,using the reconstruction image established at the prior time and of themovement field obtained in step d); ii) estimating the gamma image atthe measurement time using the estimated reconstruction image; iii)comparing the gamma image estimated at the measurement time and thegamma image acquired at the measurement time; iv) depending on thecomparison, updating the reconstruction image at the measurement time.6. The method according to claim 1, comprising determining aregistration function representative of a spatial offset between thegamma camera and the visible camera, the registration function beingused in step e).
 7. The method according to claim 1, wherein step d)comprises selecting noteworthy points in the visible images acquired atthe prior time and at the measurement time, respectively; so that themovement field comprises two-dimensional movements of the selectednoteworthy points respectively.
 8. The method according to claim 7,wherein the noteworthy points are considered to belong to the samesurface.
 9. The method according to claim 1, wherein step d) comprises:extracting first noteworthy points from the visible image acquired atthe measurement time; extracting second noteworthy points from thevisible image acquired at the prior time; matching first noteworthypoints and second noteworthy points, so as to form pairs of matchedpoints, each pair being formed from a first noteworthy point and from asecond noteworthy point; for each pair of matched points, determining amovement; obtaining the movement field on the basis of the movementsdetermined for each pair of matched points.
 10. The method according toclaim 9, comprising: generating a first mesh of the observation field,at the measurement time, using first noteworthy points, the latterforming first vertices of the first mesh; generating a second mesh ofthe observation field, at the prior time, using second noteworthypoints, the latter forming second vertices of the second mesh; dependingon the movements determined for various pairs of noteworthy points,determining, by interpolation, movements at points belonging to thefirst mesh and to the second mesh; obtaining the movement field on thebasis of the movement of each matched vertex and of the interpolatedmovements.
 11. The method according to claim 1, wherein: the device iscoupled to a movement sensor, so as to estimate a movement of the devicebetween two successive times; step d) takes into account the estimatedmovement to estimate or validate the movement field.
 12. The methodaccording to claim 1, wherein the gamma camera is configured tosimultaneously acquire, in steps a) and c), gamma images in variousenergy bands, steps b) and d) to f) being implemented for each energyband respectively, so as to obtain, at each measurement time, areconstruction image in various energy bands.
 13. The method accordingto claim 1, wherein the gamma camera is configured to simultaneouslyacquire, in steps a) and c), gamma images in various energy bands, themethod comprising a linear combination of various gamma imagesrespectively acquired, at the same measurement time, in various energybands, so as to obtain a combined gamma image, steps b) and d) to f)being carried out on the basis of a combined gamma image formed at eachmeasurement time.
 14. The method according to claim 13, wherein acombined image is formed by a weighted sum of gamma images acquired atvarious energies, the weighted sum using weighting factors, theweighting factors being determined on the basis of an emission spectrumof a radioactive isotope, such that each reconstruction image isrepresentative of a spatial distribution of the activity of the isotopein the observation field corresponding to the measurement time.
 15. Adevice, comprising: a gamma camera, comprising pixels, defining a gammaobservation field, the gamma camera being configured to determine aposition of sources of X-ray or gamma irradiation in the gammaobservation field; a visible camera, allowing a visible image of avisible observation field to be formed; the intersection of the gammaobservation field and of the visible observation field being nonzero,and defining an observation field of the device; a processing unit,configured to receive, at various measurement times: a gamma image,formed by the gamma camera; a visible image formed by the visiblecamera; wherein the processing unit is configured to carry out steps b)and d) to g) of the method according to claim 1.